[clang] Fix __builtin_object_size calculation for references of unknown origin in C++23 (PR #157778)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

In any context where the standard requires constant evaluation, we have to return false, probably.  (This corresponds, roughly, to Info.InConstantContext.  I don't think we have a bit that precisely corresponds, though.)

We probably do want to fold in tryEvaluateObjectSize, though.

https://github.com/llvm/llvm-project/pull/157778